Multi-antenna techniques are used in communication systems to improve performance. These techniques rely on multiple antennas at the transmitter and/or receiver and can be grouped into three different categories: diversity, interference suppression, and spatial multiplexing. These three categories are often collectively referred to as MIMO communication even though not all of the multi-antenna techniques that fall within these categories require at least two antennas at both the transmitter and receiver.
To provide a specific example of a multi-antenna technique, it is known that a signal transmitted over a radio channel is corrupted due to time dispersion. Time dispersion occurs when the transmitted signal propagates to a receiver over the radio channel via multiple, independently fading paths with different delays. As shown in FIG. 1, such a time dispersive channel corresponds to a non-flat channel response 102 in the frequency domain.
In the case where the transmitted signal uses a relatively wideband carrier 104, each symbol carried by the signal will be transmitted over frequencies of the time dispersive channel with both good quality (high signal strength) and bad quality (low signal strength). As a result, these symbols are said to experience frequency diversity. On the other hand, in the case where the transmitted signal uses a relatively narrowband carrier 106, such as those used in orthogonal frequency division multiplexing (OFDM) transmission, each symbol carried by the signal will experience comparatively less frequency diversity. As a result, the symbols can be confined to transmission over frequencies of the time dispersive channel with only bad quality (low signal strength), leading to a poor error-rate performance at the receiver.
The multi-antenna technique of diversity can be used to improve performance in such an instance. For example, in at least one application, diversity refers to the different fading experienced by signals transmitted from transmit antennas that are spatially separated. This diversity in fading can be exploited to create artificial frequency diversity by transmitting the same narrowband carrier 106 with different relative delays from the multiple transmit antennas. Importantly, the radio channel needs to be estimated at the receiver to take advantage of the artificially created frequency diversity when decoding the symbols carried by the narrowband carrier 106. In particular, the receiver needs to estimate the radio channel and apply its inverse response to take advantage of the artificially created frequency diversity when decoding the symbols carried by the narrowband carrier 106.
In general, many of the multi-antenna techniques that fall within the three categories mentioned above (i.e., diversity, interference suppression, and spatial multiplexing) need to perform channel estimation to improve performance. Depending on the specific multi-antenna technique implemented, the channel estimates can be required at the receiver and/or the transmitter. Channel estimation is typically performed using a training-based method where known symbols, referred to as reference symbols or pilot symbols, are transmitted to a receiver to aid in its estimation of the channel. When there are multiple transmit antennas, the signal received by a receive antenna is a superposition of the signals transmitted form each of the transmit antennas. Thus, the reference symbols transmitted from each of the multiple transmit antennas generally need to be transmitted such that they do not interfere with each other in order to accurately estimate the channel.
FIG. 2 illustrates three example reference signal patterns 200 that use different multiplexing techniques to prevent interference between reference symbols transmitted from multiple transmit antennas so that accurate channel estimation can be performed. The three example reference signal patterns 200 respectively use a time division multiplexing (TDM) technique, a frequency division multiplexing (FDM) technique, and a code division multiplexing (CDM) technique.
Using a TDM technique, reference symbols are transmitted from one transmit antenna at a time over the same frequency. An example reference signal pattern 202 that uses a TDM technique is shown at the top of FIG. 2 for two transmit antennas. During a first symbol time period t1, the top transmit antenna transmits a reference symbol over frequency f2, while the bottom transmit antenna transmits nothing over frequency f2. During a second symbol time period t2, the bottom transmit antenna takes its turn transmitting a reference symbol over frequency f2, while the bottom transmit antenna transmits nothing over frequency f2. Because the reference symbols are respectively transmitted from the two transmit antennas one at a time over frequency f2, the reference symbols are orthogonal to each other in the time domain and do not interfere with each other.
Using a FDM technique, reference symbols are transmitted from transmit antennas at the same time but over different frequencies. An example reference signal pattern 204 that uses a FDM technique is shown in the middle of FIG. 2 for two transmit antennas. During a symbol time period t1, the top transmit antenna transmits a reference symbol over frequency f3 but nothing over frequency f2, while the bottom transmit antenna transmits a reference symbol over frequency f2 but nothing over frequency f3. Because the reference symbols are respectively transmitted by the two transmit antennas over different frequencies during the same symbol time period t1, the reference symbols are orthogonal to each other in the frequency domain and do not interfere with each other.
Finally, using a CDM technique, reference symbols are transmitted from transmit antennas over the same frequency and symbol time periods using mutually orthogonal cover codes. An example reference signal pattern 206 that uses a CDM technique is shown at the bottom of FIG. 2 for two transmit antennas. During a first symbol time period t1 and a second symbol time period t2, both the top and bottom transmit antennas transmit two reference symbols (one per symbol time period) over the frequency f2. The top transmit antenna respectively applies the cover code +1, +1 (labeled as cover code 1 in FIG. 2) to the two reference symbols it transmits, and the bottom transmit antenna respectively applies the cover code +1, −1 (labeled as cover code 2 in FIG. 2) to the two reference symbols it transmits. Because cover codes 1 and 2 are mutually orthogonal, the two reference symbols transmitted from each antenna can be separated from each other, assuming the channel is substantially fixed over the symbol time periods t1 and t2.
Standardized wireless communication systems, such as 3GPP long-term Evolution (LTE), use predefined reference signal patterns to support channel estimation. These patterns typically use some combination of TDM, FDM, and potentially CDM to define some number of reference signals (i.e., signals made up of one or more reference symbols) that can each be transmitted by a different transmit antenna without interfering with one another at a receive antenna. Using the three techniques of TDM, FDM, and CDM alone, the predefined reference signal patterns of these standardized wireless communication systems often cannot be extended to define many (if any at all) additional reference signals without using additional resource elements (where a resource element corresponds to the resource of one available carrier/tone over one symbol period). Using additional resource elements is disadvantageous because these resource elements can no longer be used to carry information symbols and their addition to an existing reference signal pattern can cause backward compatibility issues with older devices operating within these systems.
However, it is expected that new releases of standardized wireless communication systems will continue to increase the number transmit antennas used to perform multi-antenna techniques, causing a corresponding need to extend their predefined reference signal patterns to define further reference signals that do not interfere with each other to perform channel estimation. For example, it is expected that the new releases of the 3GPP LTE wireless standard (release 12 and beyond) will support, for example, 16, 32, 64, or more transmit antennas at a base station for performing multi-antenna techniques, or what has been referred to as massive MIMO given the large number of antennas to be used. It is also possible that in a new release a user equipment (UE) or terminal will support more transmit antennas.
The embodiments of the present disclosure will be described with reference to the accompanying drawings. The drawing in which an element first appears is typically indicated by the leftmost digit(s) in the corresponding reference number.